U.S. embassies in a slew of Muslim countries will close on Sunday due to a credible and serious security threat.

The embassies will close in response to "a specific threat against a U.S. embassy or consulate," according to a senior U.S. official, who called it a "concerted effort" to target an embassy or consulate in a Muslim country.

"We just don't know which one," the official told ABC News.

"There could be other targets, not just embassies," another U.S. official said. The threat is considered to be throughout the Middle East, North Africa and South Asia.

Some of the biggest U.S. embassies will close, including those in Egypt, Iraq, Qatar, Bahrain and Kuwait, where tens of thousands of U.S. troops are based. The U.S. embassy in Afghanistan will also close. So will embassies in Dhaka, Bangladesh; Amman, Jordan; Muscat, Oman; Riyadh, Saudi Arabia; Algiers, Algeria; Abu Dhabi, United Arab Emirates; Sana'a, Yemen; and Tripoli, Libya, according to security warnings issued by those embassies. Two consulates in Saudi Arabia will also close, in Dharan and Jeddah, as will a consulate in Dubai, United Arab Emirates.

The State Department has instructed all U.S. embassies that would normally be open this Sunday to close, according to a senior State Department official. U.S. embassies in Muslim countries are typically open on Sundays, while others are not. Most U.S. embassies will be closed this Sunday in keeping with normal hours of operation.

The State Department announced the closings at its daily briefing on Thursday, declining to provide specifics about the security threat.

"Security considerations have led us to take this precautionary step," State Department spokeswoman Marie Harf told reporters.

The embassies could remain closed past Sunday.

While next week will mark the Muslim holiday of Eid, and the end of Ramadan, a U.S. official declined to draw any connection between the security threat and the Muslim holy calendar.

Messages were disseminated by those embassies throughout Thursday.

"The Department of State has instructed certain U.S. Embassies and Consulates to remain closed or to suspend operations on Sunday, August 4," the warnings read, providing embassy phone numbers if U.S. travelers need emergency assistance. "The Department has been apprised of information that, out of an abundance of caution and care for our employees and others who may be visiting our installations, indicates we should institute these precautionary steps. It is possible we may have additional days of closings as well, depending on our analysis."

U.S. ambassadors, not the State Department, usually determine when embassies will shut down. A senior U.S. official called today's move a "real precedent-setting event."